OBJECTIVE 4: TO UNDERSTAND THE
TYPES OF TRANSPORT MECHANISMS PRESENT IN TUBULAR EPITHELIA.
A. Net transport by passive diffusion can occur across a tubular cell membrane or across the tubular epithelium when a favorable chemical (concentration) or electrical gradient is present and the barrier has a finite permeability to the substance. Since the composition of the filtrate entering the tubule initially has practically the same composition as that of the plasma in the peritubular capillaries, active transport of some type is required to establish a gradient for passive diffusion. For example, the active reabsorption of salt in the proximal tubule drives water reabsorption which causes the concentration of urea in the tubular fluid to increase. That chemical gradient then drives passive urea reabsorption.
B. Active transport mechanisms which involve the interaction of a membrane component with the transported substance can be categorized in a number of ways:
1. Capacity-limited systems exhibit the characteristics of specificity, competition and saturation. The glucose reabsorptive system is a classic example. The system will transport only a few other sugars of similar size and they will compete among themselves for transport by the system. The capacity of the transport system is limited so that it can be easily saturated.
2. Gradient-limited systems are limited not so much by the capacity of the pump but by the chemical gradient the pump establishes. Active transport of an ion out of the tubule tends to reduce the concentration of that ion in the tubular fluid, establishing a chemical gradient for passive back-diffusion. The rate of that passive back-diffusion is affected by the electrical gradient and by the conductance or permeability of the epithelium. The difference between the rate of pumping and the rate of passive back-diffusion is the rate of net transport (Fig. 3-5).
Fig. 3-5. An illustration of a gradient-limited transport system.
The rate of passive back-diffusion is also affected by the rate of water reabsorption. A high rate tends to maintain the concentration of the ion in the tubular fluid high, retarding or preventing the establishment of a chemical gradient for an ion that is being reabsorbed. Thus, the net rate of transport will increase.
A high tubular fluid flow rate past the site of transport also tends to prevent the establishment of a chemical gradient and, therefore, permits a high rate of net transport.
3. Active transport systems can also be categorized as primary or secondary systems. A transport mechanism which directly utilizes metabolic energy is called a primary active mechanism. The Na-K pump for instance utilizes the metabolic energy stored in ATP. These systems may operate either as capacity-limited systems or as gradient-limited systems. A secondary active system utilizes the chemical or electrical energy resulting from the work of a primary transport system. The Na-H exchange mechanism for instance moves Na into the cell as a result of the Na concentration (chemical) gradient (low cell Na) established by the Na-K pump. In doing so it also transports protons out of the cell against its gradient. Actually the direction and rate of transport is governed by the algebraic sum of the gradients for both substances. The Na-H exchange mechanism is electrically neutral so the electrical gradient across the membrane has no effect on the transport. The Na-glucose cotransporter transfers net charge so that the total gradient for the system includes the sum of the chemical gradients for the two substances plus the electrical gradient for Na. These systems may also operate either as capacity-limited systems or gradient-limited systems.
4. There are additional terms applied to transport systems. Symports move two or more substances in the same direction, e.g., the Na-glucose symport operating in the apical membrane of the proximal tubule. Antiports move two substances in opposite directions, e.g., Na-H antiport.
QUESTIONS:
13.
What are the three distinguishing features of carrier-mediated
transport? Why is the characteristic of saturation unimportant in
determining the limiting rate of net Na transport?
14. What is meant by the term "gradient-limited"? How does the rate of water reabsorption affect the net rate of transport of an ion by a gradient-limited reabsorptive system? What would be the effect on Na reabsorption by the proximal tubule if water reabsorption is blocked?
15. How does the rate of flow of tubular fluid affect the net rate of Na reabsorption in the distal tubule? How would the rate of water reabsorption affect the net rate of K secretion? How would the tubular fluid flow rate affect it?
16. What is a "secondary" active transport system? A "symport"? An "antiport"? What determines the effective driving force for a secondary active transport system?
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